Under normal driving conditions it is generally desirable for the amount of body roll allowed by an active roll control system to be sufficient to signal to the driver when the vehicle is cornering hard. The system therefore needs to allow increasing body roll as the lateral acceleration increases. A system which does this is described in WO98/26948. However in some circumstances, in particular when the vehicle travels through a series of bends in a slalom type motion at a certain frequency at or near the resonant frequency of vibration of the vehicle about its roll axis, the magnitude of the roll vibrations can build up, the normal roll control characteristics being insufficient to prevent this.
U.S. Pat. No. 4,624,477 and U.S. Pat. No. 4,634,143 disclose semi-active damper systems in which the damping rate of a switchable damper is increased to reduce build up of body roll if slalom-type driving is detected.
Accordingly the present invention provides a suspension system for a vehicle the system comprising actuation means for applying forces between sprung and unsprung parts of a vehicle to control vehicle roll, lateral acceleration measuring means for providing a measure of the magnitude and an indication of the direction of lateral acceleration of the vehicle, and control means arranged to monitor said measure and to control the actuation means in a normal mode to provide an actuator force, dependent on said lateral acceleration, resisting vehicle roll, characterized in that the control means is further arranged, if the lateral acceleration changes direction a predetermined number of times, at a frequency which is within a predetermined frequency range, and the measure of lateral acceleration during each turn exceeds a predetermined level to enter a slalom mode in which it controls the actuation means to provide, for a particular lateral acceleration, more resistance to body roll than in the normal mode, at least over a range of lateral accelerations.
The lateral acceleration may be measured directly using a lateral accelerometer, or indirectly by measuring, for example, the steering angle of the steered wheels of the vehicle and the vehicle speed.
The actuation means may form part of an active roll-bar assembly and be arranged to apply a torque between the two ends of an anti-roll bar in order to control vehicle roll. Alternatively the actuation means may comprise a set of pneumatically or hydraulically actuated struts, one for each wheel of the vehicle, the struts being controlled to raise one side of the vehicle body and lower the other to control vehicle roll.
Preferably the control means is arranged in the slalom mode to control the actuation means so as substantially to prevent body roll up to the maximum force available from the actuation means.
Preferably the control means is arranged in normal operation to control the actuation means so as substantially to prevent body roll up to a predetermined level of lateral acceleration, and then to allow a controlled level of body roll for higher lateral accelerations.
Preferably, up to said predetermined level of acceleration, the anti-roll force produced by the actuators is the same in slalom mode as in normal operation, and the control means is arranged to change between normal operation and slalom mode only when the lateral acceleration is below said predetermined level.